Methods for Controlling Syngas Composition

This continuation application claims the benefit of U.S. patent application Ser. No. 17/543,905, filed on Dec. 7, 2021 which claims the benefit of U.S. Provisional Application Ser. No. 63/141,046, filed on Jan. 25, 2021, which is incorporated herein by reference.

The present invention relates to the production of syngas so as to control significant characteristics of the syngas so produced.

Primary gasification is often used in industry to convert a feedstock to a syngas stream containing CO and/or Hby partial oxidation. A primary gasifier consists of a vessel, typically refractory lined, where a primary feedstock is mixed with an oxidant stream. Common oxidant streams include steam, CO2, oxygen, or mixtures of these streams. Depending on the source of the oxidant other species may also be included, such as N2 or Ar. The ratio of oxidant to feedstock is controlled such that less oxidant is provided than required to completely combust the feedstock. This condition, termed “fuel rich”, leads to the production of desired species such as CO and H2 by partial oxidation. The resulting crude syngas is typically then purified and sent to a downstream process for use. Examples of downstream processes include methanol production and Fischer-Tropsch (“FT”) processes for liquid fuels production.

In some cases the syngas produced by primary gasification may contain significant amounts of unreacted higher molecular weight hydrocarbons which can be problematic for downstream equipment. One example of problematic hydrocarbons is those commonly denoted as “tars” that condense in downstream equipment potentially causing operational and efficiency issues. These problematic hydrocarbons can be further processed by secondary gasification of the hydrocarbon-containing syngas from a primary gasifier. This configuration is similar to a primary gasifier except that the feedstock to the secondary gasifier includes, at least in part, the crude syngas from the primary gasifier. A secondary gasifier may be used with feedstocks generated from hydrocarbon processing, such as refinery off gas (that is, crude syngas is not necessarily generated from a gasification process).

A gasification process is particularly suited for chemicals manufacturing. H2 and CO are converted to chemicals using a variety of processes, including catalytic or biological reactors. To optimize the efficiency of the chemical generating reactors, syngas from a gasification system is conditioned in any of several ways; a partial list of potential conditioning actions is given below. Each conditioning step increases the operating complexity as well as capital and operating cost of the overall chemical plant, so plants limit the number of conditioning steps to only those required.

Depending on the chemical being produced, different syngas properties are required to maximize efficiency. For example, production of transportation fuels using a Fischer-Tropsch system is most efficient with feeds having H2:CO ratios in the range of 1.95 to 2.05. The native H2:CO ratio of a gasification system may not fall within the range required by the downstream process. For example, the native H2:CO ratio of products formed by partial oxidation (POx) gasifiers using natural gas (“NG”) as a feedstock fall within the range of 1.7 to 1.8. If NG is being converted to syngas using a POx gasifier and the syngas is intended to be used to generate ethanol using FT processing, the H2:CO ratio of this syngas will preliminarily be adjusted upward using a WGS reactor. Because of the many types of gasifiers, feedstocks, chemical conversion processes and chemicals, it is recognized that linking the gasification process to the chemical product generation process will usually require adjustment of the H2:CO ratio.

Adjusting the H2:CO ratio in syngas produced by gasifiers such as POx reactors has previously been accomplished by adding into a reactant stream that is fed into the POx reactor, either H2O in the form of steam for situations where a higher H2:CO ratio is desired or a CO2 rich stream when a reduction in H2:CO ratio is desired. (For example, a source of CO2 may be a CO2 stream obtained by a removal process in the conditioning steps.) This is done primarily in steam methane reformers (SMR) but is also applied to a lesser extent with auto thermal reformers (ATR) or even to a lesser extent with partial oxidation reformers.

The present invention utilizes discoveries that enable the control of the characteristics of the syngas which is produced in the POx reactor, that provide advantages in being able to control the characteristics of the syngas.

One embodiment of the present invention comprises a method of treating a syngas stream, comprising

Preferably the temperature reduction of (B1) is carried out according to a time temperature history s described herein that lowers the temperature at a sufficiently high rate that the H2:CO ratio is modified as desired and is then maintained at a new modified value.

Preferably the addition of steam is provided in a location near the gasifier exit and/or high temperature ductwork connecting the gasifier to the syngas cooler, and preferably provides at least 1 second (preferably up to 5 seconds) of residence time before entering any downstream syngas cooler.

Another embodiment of the present invention comprises a method of treating a syngas stream, comprising

Preferably the addition of carbon dioxide is provided in a location near the gasifier exit and/or high temperature ductwork connecting the gasifier to the syngas cooler, and preferably provides at least 1 second (preferably up to 5 seconds) of residence time before entering any downstream syngas cooler.

The present invention is particularly useful in operations that convert hydrocarbon products such as biomass to useful hydrocarbon products such as (but not limited to) liquid fuel. The feedstock produced by the present invention includes products that can be sold and used as-is, as well as products that can be used as reactants to produce other finished useful products that can then be sold and used.

is a flowsheet that shows the typical steps of such an operation.

Referring to, streamwhich is also referred to herein as the raw feedstock is fed to partial oxidation reactor. Streamis provided from sourcewhich designates a production facility or reactor in which raw feedis produced.

Examples of suitable raw feedstocksand their sourcesinclude:

Raw feedstockgenerally contains hydrogen and carbon monoxide (CO), and typically also contains one or more hydrocarbons such as alkanes and/or alkanols of 1 to 18 carbon atoms, and often contains one or more of carbon dioxide (CO), and higher molecular weight hydrocarbons characterized as tars and/or soot.

The raw feedstock stream, if heated as it leaves source, typically exhibits a temperature of between about 500° F. and 1600° F.

Raw feedstock streamis then fed into partial oxidation reactorin which it is reacted (under conditions described more fully below) with oxygen that is provided as hot oxygen stream(produced as more fully described below) to produce additional amounts of hydrogen and carbon monoxide (CO) from components present in stream. If tars are present in the stream, some or all of tars present can also be converted to lower molecular weight hydrocarbon products.

Oxidized product streamwhich is produced in partial oxidation reactoris fed to stagein which streamis preferably cooled and treated to remove substances that should not be present when the stream is fed to reactor(described hereinbelow). Stagetypically includes a unit which cools stream, for instance by indirect heat exchange with incoming feed waterto produce streamof heated water and/or steam. In alternative embodiments, stagecan also comprise a shift conversion reactor in which carbon monoxide in streamis reacted (in a non-limiting example, with water vapor (steam)) in a catalytically mediated water-gas shift (“WGS”) reaction to produce hydrogen, thereby providing a way to adjust the ratio of hydrogen to carbon monoxide in stream. The heat removal in stageand its beneficial advantages are described more fully below. The heat removal in stageis performed before any other treatment or reaction of the syngas.

The resultant stream, having been cooled and/or having had its hydrogen:CO ratio adjusted in stage, is fed to stagein which impuritiesthat may be present such as particulates, acid gases including CO, ammonia, sulfur species, and other inorganic substances such as alkali compounds, are removed. Impurities may be removed in one unit or in a series of units each intended to remove different ones of these impurities that are present or to reduce specific contaminants to the desired low levels. Stagerepresents the impurities removal whether achieved by one unit or by more than one unit. Cooling and impurities removal are preferably performed in any effective sequence in a series of stages or all in one unit. Details are not shown but will be familiar to those skilled in the art. Stagetypically includes operations for final removal of impurities, non-limiting examples of which include particulates, NH, sulfur species and CO. The COremoval is typically performed by a solvent-based process, which either uses a physical solvent, e.g. methanol, or a chemical solvent, e.g. amine.

The resulting cooled, conditioned gaseous streamis then fed to stagewhich represents any beneficial use of one or more components present in stream. That is, streamcan be used as-is as an end product. However, the present invention is particularly useful when streamis to serve as feedstock for further reaction and/or other processing that produces product designated asin.

One preferred example of such further processing is conversion of streaminto liquid fuels, such as using streamas feed material to a Fischer-Tropsch process or other synthetic methodology to produce a liquid hydrocarbon or a mixture of liquid hydrocarbons useful as fuel.

Other examples of useful treatment of streaminclude the production of specific targeted chemical compounds such as ethanol, straight-chain or branched-chain or cyclic alkanes and alkanols containing 4 to 18 carbon atoms, aromatics, and mixtures thereof; or in the production of longer-chain products such as polymers.

The overall composition of streamcan vary widely depending on the composition of raw feedstock, on intermediate processing steps, and on operating conditions. Streamtypically contains (on a dry basis) 20 to 50 vol. % of hydrogen, and 10 to 45 vol. % of carbon monoxide.

However, it is preferred that one or more properties of streamwill continually exhibit a value, or a value that falls within a characteristic desired range, in order to accommodate the treatment that streamis to undergo in stageto produce a repeatable, reliable supply of product.

In a preferred practice of the present invention, the property of streamthat is relevant and that should be maintained within a desired ratio, is the molar ratio of hydrogen (H) to CO. For FT fuels production, the target range of H:CO molar ratio depends on the product being produced. For example, ethanol production is most efficient with H:CO within the range of 1.95 to 2.05. Synthetic gasoline production requires a H:CO ratio in the range of 0.55 to 0.65. For fuels production by other conversion mechanisms, such as biological conversion, the target range of H:CO molar ratio can be very large. According to the Wood-Ljungdahl pathway, depending on the type of bacteria being used, streams containing only CO, only Hor any combination of H:CO can be utilized due to the bacteria's ability to convert HO and COinto Hand CO as needed. Each bacterial strain will prefer a particular chemical makeup of syngas at which it is most efficient in producing the desired product.

Referring again to, processing in stagemay produce byproduct stream, which can be recycled to partial oxidation reactorto be used as a reactant, and/or recycled to hot oxygen generator(described below with respect to) to be combusted in hot oxygen generatoras described herein. Steam (stream) formed from water streamin stagecan be optionally fed to partial oxidation reactor.

Referring to, hot oxygen streamis fed to partial oxidation reactorto provide oxygen for the desired partial oxidation of raw feedstock, and to provide enhanced mixing, accelerated oxidation kinetics, and accelerated kinetics of the reforming with reactor.

There are many ways in which the desired high temperature, high velocity oxygen-containing stream can be provided, such as plasma heating.

One preferred way is illustrated in, namely hot oxygen generator, that can provide hot oxygen streamat a high velocity. Streamof gaseous oxidant preferably having an oxygen concentration of at least 30 volume percent and more preferably at least 85 volume percent is fed into hot oxygen generatorwhich is preferably a chamber or duct having an inletfor the oxidantand having an outlet nozzlefor the streamof hot oxygen. Most preferably the oxidantis technically pure oxygen having an oxygen concentration of at least 99.5 volume percent. The oxidantfed to the hot oxygen generatorhas an initial velocity which is generally within the range of from 50 to 300 feet per second (fps) and typically will be less than 200 fps.

Streamof fuel is provided into the hot oxygen generatorthrough a suitable fuel conduitending with nozzlewhich may be any suitable nozzle generally used for fuel injection. The fuel may be any suitable combustible fluid examples of which include natural gas, methane, propane, hydrogen and coke oven gas, or may be a process stream such as streamobtained from stage. Preferably the fuelis a gaseous fuel. Liquid fuels such as numberfuel oil or byproduct streammay also be used.

The fuel in streamand the oxidant streamshould be fed into generatorat rates relative to each other such that the amount of oxygen in oxidant streamconstitutes a sufficient amount of oxygen for the intended use of the hot oxygen stream. The fuelprovided into the hot oxygen generatorcombusts therein with oxygen from oxidant streamto produce heat and combustion reaction products which may also include carbon monoxide.

The combustion within generatorgenerally raises the temperature of remaining oxygen within generatorby at least about 500° F., and preferably by at least about 1000° F. The hot oxygen obtained in this way is passed from the hot oxygen generatoras streaminto partial oxidation reactorthrough and out of a suitable opening or nozzleas a high velocity hot oxygen stream having a temperature of at least 2000° F. up to 4700° F. Generally the velocity of the hot oxygen streamas it passes out of nozzlewill be within the range of from 500 to 4500 feet per second (fps), and will typically exceed the velocity of streamby at least 300 fps. The momentums of the hot oxygen stream and of the feedstock, should be sufficiently high to achieve desired levels of mixing of the oxygen and the feed. The momentum flux ratio of the hot oxygen stream to the feedstock stream should be at least 3.0.

The composition of the hot oxygen stream depends on the conditions under which the stream is generated, but preferably it contains at least 50 vol. % Oand more preferably at least 65 vol. % O. The formation of the high velocity hot oxygen stream can be carried out in accordance with the description in U.S. Pat. No. 5,266,024.

It will be recognized that the desired state of systems that employ partial oxidation in the course of producing hydrocarbon feedstock is this: that there is little or no perturbation of the characteristics of the raw feedstock, of the oxygen stream, or of streams,and, nor of the operating conditions employed in the partial oxidation reactorand in stagesand. In addition, circumstances may arise in which characteristics of raw feedstockto the POx reactor change in a way such that, if nothing else changes in the operating conditions, the characteristics of streamorwould be changed in a manner that would adversely affect the characteristics of the desired product stream. Such a change in streamis, of course, undesirable.

Alternatively, it will also be recognized that the characteristics of the product to be formed in stageare required to change, necessitating a change on the H2:CO ratio of the syngas at.

The characteristics of raw feedstockthat could change include the total hydrocarbon concentration of the raw feedstock; the total concentration of CH, CH, and tars; and the temperature. Examples of circumstances that could cause any of these characteristics to change include:

The composition of raw feedstockhas changed because the feed to sourcehas changed.

The raw feedstockfrom its sourcehas become too expensive relative to other compositions, from other sources, that could be useful feedstock material to the POx reactor.

The treatment provided in one or more of the stagesandhas changed, such as changes to the catalytic processing that is provided in the WGS reaction.

The injector system that feeds material into the POx reactor has been damaged or fouled so that the ability of the feedstock to be entrained into the hot oxygen stream is lessened, thereby leading to excessive methane slip, excessive tar slip, and/or excessive soot formation.

In the past, customary practice to accommodate changes in circumstances such as these, which involve changes to characteristics of the raw feedstockto POx reactoror changes to the desired product of, has often been shutting down the overall facility, or at best running the facility at a partial load which is detrimental to capital recovery. When that occurs, an operator who has more than one such facility must then rely on the output of product that is available from other facilities, or else suffer the loss of production.

It has been found however that the present invention enables the operator to adjust the H2:CO ratio of the syngas product that emerges from the POx reactor, to compensate for any changes in the overall operation that would require adjustment of the H2:CO ratio of that product.

This invention improves the syngas conditioning capability of a chemical plant by controlling the H2:CO ratio in the syngas streamimmediately downstream of the gasifier or POx reactor. This ability results in a reduction in size or potentially eliminating a WGS reactor (or reverse WGS if a lower H2:CO ratio is needed). This in turn reduces the amount of catalyst needed for initial charging and for replacement.

In this invention, heat energy is removed from the syngasto reduce temperature to a level acceptable for downstream conditioning operations. Removing energy changes the equilibrium composition of the mixture, specifically impacting the relative amounts of H2, H2O, CO and CO2 according to the water gas shift reaction CO+H2O<=>H2+CO2. The difference between the actual concentration of each component and the equilibrium concentration represents a chemical driving force, moving the system toward equilibrium over time.

The rate at which temperature of the stream is lowered impacts the composition of the syngas. At higher temperatures, the syngas retains sufficient energy to overcome kinetic limitations allowing the reaction in the syngas to proceed long enough to produce meaningful change in the composition. Once a syngas is below a certain temperature, even though the driving force for increasing the H2:CO ratio still exists, there is insufficient energy in the gas to promote the reactions, so that the H2:CO ratio of the syngas is “frozen” or “quenched”.

As an alternate embodiment of this invention, streamof steam or CO2 is added to streamnear the exit of the POx reactor. The temperature of streamis sufficient to enable reactions changing the H2:CO ratio to proceed significantly in reasonable residence times within which the temperature is lowered, potentially in as little as 1 second but preferably within up to 5 seconds, with the temperature having been lowered by the end of this period of time to a temperature at which the H2:CO ratio no longer changes. By modulating the amount of steam(or CO2, in the alternative embodiment described elsewhere herein) being added to streamit is possible to obtain a targeted value of H2:CO. For example, if the product being produced by the plant is made most efficiently with a H2:CO ratio of 2.0, steam is added in an amount that maintains the H2:CO ratio. If conditions either upstream or downstream of the POx reactor change, for example if the feedstock to the POx reactor changes in composition or temperature, the steam amount can be adjusted to maintain the H2:CO ratio at 2.0 without making any equipment or other process modifications. Another example is if a different productwill be made, it is likely the optimum H2:CO ratio will be different. That is, by adjusting the amount of steamadded to the streamat the exit of reactor, or even changing from steam addition to CO2 addition (or vice versa), the H2:CO ratio can be adjusted in the POx system to match the target composition of the syngas in stream.

The injection of H2O or CO2 into streamnear the exit of reactorlimits the participation of those components to water gas shift chemistry and not the reforming chemistry. This results in a higher overall H2+CO rate and lower feedstock and O2 rates, resulting in higher productivity and lower operating costs. Additionally, the syngas entering the syngas cooler is at a lower temperature which will increase the syngas cooler lifetime.